Tuesday, June 24, 2008

Dating from Magnetic Particles

This AP article is about American archaeology, but the technology could be applied to the Near East, if it hasn't been already.

You might be surprised what you can learn from a campfire. A campfire that has been cold for, say, 300 years.

Stacey Lengyel hopes she can tell, within 30 years or so, when it was used.

Lengyel, a research associate in anthropology at the Illinois State Museum, is the country's leading authority on archeomagnetic dating, a process built around two phenomena: when heated, magnetic particles reorient themselves to magnetic north; and over time, magnetic north is, literally, all over the map.

"They call it a 'drunken wander,' " said Lengyel. "Around 1600, it was real close to Earth's rotational axis. Now, it is around 75 degrees latitude...."

In archeomagnetic dating, once potential samples have been identified, their location and orientation are precisely measured, Lengyel said. About a dozen 1-inch cubes are then excised, encased to preserve them, then taken to a lab.

The chunks are then progressively demagnetized until their natural remnant magnetism can be measured, she said. The objects may have been partially magnetized by nearby lightning strikes, for example, or if they were stored near objects with strong magnetic fields. These weaker magnetic fields must be removed.

First their magnetic fingerprint is taken, and then they are slightly demagnetized. The process is repeated several times; eventually all that is left is the baseline magnetic signal, she said. If the material is fired to about 500 degrees Celsius or more, the magnetic field will point to where magnetic north was located at the time.

"The best dates we can get are within a 30-year time period," Lengyel said.